Nonvolatile memory cells with buried channel transistors

ABSTRACT

In a nonvolatile memory cell ( 110 ), the select gate transistor is formed as a buried channel transistor to increase the transistor current.

BACKGROUND OF THE INVENTION

The present invention relates to nonvolatile memories.

FIGS. 1–4 illustrate a flash memory fabrication process described in vanDuuren et al., “Compact poly-CMP embedded flash memory cells for one ortwo bit storage”, Proceedings of NVSMW 2003 (Non-Volatile SemiconductorMemory Workshop), Monterey, Calif., pages 73–74. Tunnel oxide 150,polysilicon floating gate 160, inter-poly dielectric 164, control gate170, and a nitride cap layer 172 are fabricated in a stacked structure(“FG/CG stack”). TEOS spacers 176 are formed on both sides of the stack.Then oxide 130 is grown for the access gate.

AG (access gate) polysilicon 140 is deposited over the FG/CG stack. SeeFIG. 2. Polysilicon 140 is polished by chemical mechanical polishing(CMP), as shown in FIG. 3. Then polysilicon 140 is patterned usingresist 173 to define the access gate, as shown in FIGS. 3 and 4.Source/drain regions 174 are formed to obtain a one-bit memory cell 102(FIG. 4).

As noted in the Duuren et al. article, the length of access gate 140depends on the mask alignment, “which could lead to an odd-even wordline effect in arrays”.

FIG. 5 shows a two-bit memory cell 110 described in the same article.Two FG/CG stack transistors 110L, 110R share an access gate 140.According to the Duuren et al. article, the cell is fabricated with thesame process as cell 102, but cell 110 is fully self-aligned andtherefore not sensitive to mask misalignment.

Each bit 110L, 110R can be programmed or erased independently of theother bit. The bit can be programmed by Fowler-Nordheim tunneling (FN)or source side injection (SSI). The Duuren et al. article states thatthe two bit cell has been studied “with 180 bit arrays in a virtualground configuration”. The read, program (SSI) and erase voltages bit110R are shown respectively in FIGS. 6, 7 and 8. In the read and programoperations (FIGS. 6 and 7), the “pass” voltage for the control gate inbit 110L (6.0 V) is high enough to turn on the corresponding FG/CGtransistor regardless of the state of its floating gate.

In the cells of FIGS. 4 and 5, the channel region of the floating gatetransistors (the FG/CG transistors) and the channel region of the accessgate transistor are merged into a single channel extending between thesource/drain regions 174. The long channel is beneficial for the cellscalability.

It is desirable to provide improved memory cells.

SUMMARY

This section summarizes some features of the invention. Other featuresare described in the subsequent sections. The invention is defined bythe appended claims which are incorporated into this section byreference.

It is generally desirable to increase the cell current to speed up thecell operations (e.g. reading or programming). Some embodiments of thepresent invention provide ways to increase the current through theaccess gate transistor (also called “select gate transistor” below).

It is well known that a PMOS transistor current is increased if thetransistor has an N+ doped polysilicon gate and the channel iscounterdoped near the surface with a P type impurity (boron). Such PMOStransistors are called buried channel (BC) transistors because thesetransistors have more current flowing below the channel surface than atthe channel surface. A disadvantage of the BC PMOS transistors is thatthey are more susceptible to the short channel effects. See Wolf,“Silicon Processing for the VLSI Era”, vol. 3 (“The Submicron MOSFET”)1995, pages 288–311, incorporated herein by reference.

The inventor has observed that if a memory cell has a longer channelobtained by merging together the channel of the floating gate transistorand the channel of the select transistor, the short channel effects areless of a problem, so a buried channel transistor becomes an attractiveway to increase the transistor current.

While the Wolf reference cited above relates to the PMOS transistors, itis believed that a similar effect (a larger current drive) should beobtainable with an NMOS transistor having a type P gate and a channelregion counterdoped near the surface with an N type impurity.

Other features and advantages of the invention are described below. Theinvention is defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1–8 shows vertical cross sections of prior art memory cells andintermediate structures obtained in prior art fabrication processes.

FIG. 9 is a circuit diagram of a memory array according to an embodimentof the present invention.

FIG. 10A is a top view of a memory array according to an embodiment ofthe present invention.

FIG. 10B is a perspective view showing some features of the memory ofFIG. 10A.

FIGS. 11, 12A, 12B, 13A, 13B, 14A, 14B show vertical cross sections ofintegrated circuit structures according to embodiments of the presentinvention.

FIG. 15 is a perspective view of an integrated circuit structureaccording to an embodiment of the present invention.

FIGS. 16, 17, 18, 19A, 19B, 20A, 20B, 21A, 21B, 22, 23A, 23B, 24,25,26A, 26B, 27A, 27B, 28, 29A, 29B, 30A, 30B, 31A–31D show vertical crosssections of integrated circuit structures according to embodiments ofthe present invention.

FIGS. 31E, 32 are top views of integrated circuit structures accordingto embodiments of the present invention.

DESCRIPTION OF SOME EMBODIMENTS

The embodiments described in this section illustrate but do not limitthe invention. The invention is not limited to particular materials,process steps, or dimensions. The invention is defined by the appendedclaims.

One embodiment of the invention will now be described on the example ofthe memory array of FIG. 9. In this example, the array has 4 rows and 5columns, but any number of rows and columns can be present. FIG. 10A isa top view of the array. FIG. 10B is a perspective view. Each memorycell 110 may have the same structure is in FIG. 5, but may also have adifferent structure (see e.g. FIG. 30A). Each cell 110 has two FG/CGstacks per one select gate 140. Conductive select gate lines 140 andconductive control gate lines 170 run through the memory array in the Ydirection (row direction). Each row includes one select gate line 140and two control gate lines 170. The line 140 provides the select gatesfor that row of cells. One of the lines 170 provides the control gatesfor the bits 110L in that row, and the other line 170 provides thecontrol gates for the bits 110R. Bitlines 180 (marked BL0–BL5 for rows0–5) run in the X direction (column direction). The bitlines contact thecorresponding source/drain regions 174 (“bitline regions”) in areas 174C(FIG. 10A) marked with a cross. Floating gates 160 are marked withdashed crosses in FIG. 10A. The floating gates can be completelyself-aligned (i.e. defined independently of photolithographicalignment), as described below.

Substrate isolation trenches 220T run through the array in the Xdirection. Trenches 220T are filled with dielectric 220 (fieldisolation). Active areas 222 run through the array between the trenches220T. Each active area 222 includes active areas of individual cells inone memory column. The active area of each cell consists of the cell'ssource/drain regions 174 and the P type channel region extending betweenthe regions 174.

In each column, each two consecutive memory cells have their adjacentsource/drain regions 174 merged into a single contiguous region(referenced by the same numeral 174). Each such region 174 provides thesource/drain regions to only two of the memory cells in each column. Ineach column 1–4 (each column except the first column and the lastcolumn), each source/drain region 174 is connected to a source/drainregion 174 of an adjacent column. The connections alternate, e.g. onesource/drain region 174 in column 1 is connected to a source/drainregion 174 in column 0, the next region 174 in column 1 is connected toregion 174 in column 2, the next region 174 in column 1 is connected toregion 174 in column 0, and so on. Bitline BL1 (column 1) is connectedto those regions 174 of column 1 that are connected to column 0; bitlineBL2 is connected to those regions 174 in column 1 that are connected tocolumn 2, and so on. Bitlines BL0 and BL5 are each connected to only onecolumn. In some embodiments, these two bitlines are shorted together.

As shown in FIG. 10A, the source/drain regions 174 of each column areseparated from the source/drain regions 174 in the adjacent columns byfield isolation regions 220.

Some of the figures below illustrate vertical cross sections ofintermediate structures obtained during the memory fabrication. Thesectional planes are indicated in FIG. 10A by lines X1–X1′, X2–X2′,Y1–Y1′, and Y2–Y2′. The line X1–X1′ runs in the X direction throughfloating gates 160 (through an active area 222). The line X2–X2′ runs inthe X direction between the floating gates (through a trench 220T). Theline Y1–Y1′ runs in the Y direction through a select gate line 140. Theline Y2–Y2′ runs in the Y direction through a control gate line 170 andfloating gates 160.

In one embodiment, the memory is fabricated as follows. Substrateisolation regions 220 are formed in P doped substrate 120 by shallowtrench isolation technology (“STI”). See FIG. 11 (cross section Y1–Y1′).Each region 220 is a dielectric region formed in a trench 220T. SuitableSTI processes are described in U.S. Pat. No. 6,355,524 issued Mar. 12,2002 to Tuan et al.; U.S. patent application Ser. No. 10/262,785 filedOct. 1, 2002 by Yi Ding; and U.S. patent application Ser. No. 10/266,378filed Oct. 7, 2002 by C. Hsiao, all incorporated herein by reference.Other STI and non-STI processes are also possible. Dielectric 220 issometimes called “STI oxide” hereinbelow because it is silicon dioxidein some embodiments. The invention is not limited to such embodiments orto silicon integrated circuits.

Substrate isolation regions are also formed in the memory peripheralarea (not shown in FIG. 11). The peripheral area contains circuitryneeded to access the memory, and may also contain unrelated circuitry(the memory may be embedded into a larger system).

As shown in FIG. 11, oxide 220 protrudes above the substrate 120. Theprotruding portions are shown at 220P. An exemplary thickness ofportions 220P is 0.12 μm for a 0.18 μm fabrication process (a processwith a 0.18 μm minimum line width). The exemplary dimensions given inthis section assume a 0.18 μm fabrication process unless mentionedotherwise.

Dopant is implanted into substrate 120 to form an N type region 604underlying the memory array. Dopant is also implanted into the substratearound the array to form a surrounding N type region (not shown)extending from the top surface of substrate 120 down to region 604.These implants create a fully isolated P well 120W for the memory array.Region 604 is not shown in the subsequent drawings, and the P well 120Wis shown simply as substrate 120.

Ion implantation steps (“Vt adjust implants”) may be performed into theactive areas of substrate 120 to adjust the transistor thresholdvoltages as needed. One such implant is an N type implant (e.g. arsenic)performed into the array to reduce the threshold voltage of the selectgate transistors. This implant creates a counterdoped region 230 at thesurface of substrate 120. Region 230 may remain type P, but the net Ptype dopant concentration in this region is reduced. It is believed thatthe select transistor (the transistor with gate 140 in FIG. 5) becomes aburied channel transistor rather than a surface channel transistor, andthe transistor current is increased. In some embodiments, the depth ofregion 230 is 0.05 μm to 0.1 μm. In some embodiments, the depth is somenumber not greater than 0.2 μm, as described for the buried channel PMOStransistors in Wolf, “Silicon Processing for the VLSI Era”, vol. 3 (“TheSubmicron MOSFET”) 1995, pages 288–311, incorporated herein byreference. Other depth values are also possible. The depth of region 230is the depth at which the net N type dopant concentration reaches itsminimum.

Silicon dioxide 130 (FIG. 12A, cross section Y1–Y1′, and FIG. 12B,periphery) is thermally grown on the exposed areas of substrate 120 toprovide gate dielectric for the select gates of the memory array and forthe peripheral transistors. An exemplary thickness of oxide 130 in thearray area is 120 Å. Generally, the oxide thickness depends on themaximum voltage that the oxide 130 is designed to sustain during thememory operation. Oxide 130 can be nitrided when it is being grown, orafter it has been grown, to impede boron diffusion from floating gates160 into substrate 120.

In the example shown in FIG. 12B, the peripheral area includes a highvoltage transistor area 512H and a low voltage transistor area 512L.Oxide 130 is grown thermally to a thickness of 60 Å over the entirewafer. This oxide is removed from the low voltage area 512L by a maskedetch. The wafer is re-oxidized to re-grow silicon dioxide in area 512Lto a thickness of 60 Å. The oxide thickness in the memory array area andin high voltage area 512H increases from 60 Å to 120 Å during this step.

Thus, oxide 130 in the array area and oxide 130 in the high voltageperipheral area 512H is formed simultaneously in these two oxidationsteps. All of oxide 130 in area 512L and part of the oxide 130 in thearray area and area 512H are formed simultaneously in the secondoxidation step.

As shown in FIG. 13A (cross section Y1–Y1′) and FIG. 13B (periphery),intrinsic polysilicon layer 140 is formed over the structure by aconformal deposition process (e.g. low pressure chemical vapordeposition, “LPCVD”). Polysilicon 140 fills the spaces between the oxideprotrusions 220P in the memory array area. The top polysilicon surfaceis planar because the polysilicon portions deposited on the sidewalls ofprotrusions 220P meet together.

FIG. 13B may represent either the low voltage or the high voltagetransistor area. In some embodiments, there are more than two peripheralareas with different gate oxide thicknesses, and FIG. 13B may representany of these areas.

Polysilicon 140 covers the regions 120 i (FIG. 13B) at the interfacebetween substrate 120 and field oxide 220 in the peripheral area.Polysilicon 140 will protect the oxide 220 in this area to preventformation of grooves (“divots”) during subsequent processing.Polysilicon 140 will be used to form the peripheral transistor gates.The grooving in regions 120 i under the transistor gates is undesirablebecause it degrades the transistor characteristics.

Layer 140 can also be formed by non-conformal deposition processes,whether known or to be invented. If the top surface of polysilicon 140is not planar, it is believed that the polysilicon 140 can be planarizedusing known techniques (e.g. CMP, or spinning a photoresist layer overthe polysilicon 140 and then simultaneously etching the resist and thepolysilicon at equal etch rates until all of the photoresist isremoved). The bottom surface of polysilicon 140 is non-planar as it goesup and down over the oxide protrusions 220P.

An exemplary final thickness of polysilicon 140 is 0.16 μm over theactive areas.

The peripheral area is masked, and polysilicon 140 is doped P+ in thearray area. Polysilicon 140 remains undoped (“INTR”, i.e. intrinsic) inthe periphery. The peripheral transistor gates will be doped later, withthe NMOS gates doped N+ and the PMOS gates P+, to fabricate surfacechannel transistors in the periphery with appropriate thresholdvoltages. The invention is not limited to the surface channeltransistors or any peripheral processing. In particular, entirepolysilicon 140 can be doped N+ or P+ after the deposition or in situ.

Silicon dioxide 810 is deposited on polysilicon 140, by CVD (TEOS) orsome other process, to an exemplary thickness of 1500 Å. Layer 810 canalso be silicon nitride, silicon oxynitride (SiON), or some othermaterial. Layer 810 is sufficiently thick to withstand subsequent oxideetches (and in particular the etch of STI oxide 220 described below inconnection with FIG. 20A) and to protect the select gates 140 fromcounterdoping during subsequent doping steps.

In some embodiments, the top surface of polysilicon 140 and/or oxide 810is not planar.

The wafer is coated with a photoresist layer 820. See FIG. 14A, crosssection X1–X1′, and FIG. 14B, periphery. (FIG. 14B shows only the activearea, not the field oxide 220.) Resist 820 is patterned to define theselect gate lines 140. The peripheral area is covered by the resist. Thememory array geometry is not sensitive to a misalignment between mask820 and the mask defining the isolation trenches 220T (FIGS. 10A, 10B)except possibly at the boundary of the memory array.

Silicon dioxide 810 is etched through the resist openings. The resist isremoved, and polysilicon 140 is etched away where exposed by oxide 810.Then the exposed oxide 130 is removed. (In an alternative embodiment,the resist 820 is removed after the etch of polysilicon 140 and/or oxide130.) The select gate lines are formed as a result. Each select gate 140will control the conductivity of the underlying portion of the cell'schannel region in substrate 120. FIG. 15 is a perspective view of theresulting structure in the array area.

The etch of polysilicon 140 can be a perfectly anisotropic verticaletch. Alternatively, the etch can have a horizontal component to reducethe width Ls (FIG. 14A) of select gate lines 140 (the width Ls is thechannel length of the select gate transistor). In one embodiment, aperfectly vertical etch is performed first to remove the exposedportions of layer 140, and then an isotropic etch is performed to reducethe width Ls.

In another embodiment, one or more etching steps are performed asdescribed above to form the lines 140. Then the sidewalls of lines 140are oxidized. Substrate 120 is also oxidized in this step. The selectgate line width Ls is reduced as a result. Then the oxide is removed.

The width Ls can also be reduced by a horizontal etch of layer 810.E.g., if layer 810 is SiON, a dry etch having a horizontal component canbe used to pattern this layer.

In another embodiment, the sidewalls of the select gate lines arereacted with some material other than oxygen, with a reaction productforming on the sidewalls. The reaction product is then removed.

The lines 140 can thus be more narrow than the minimal photolithographicline width. The memory packing density is therefore increased.

As shown in FIG. 16 (cross section X1–X1′), the structure is oxidized togrow silicon dioxide 150 on substrate 120 and the sidewall surfaces ofpolysilicon gates 140 in the array area. Oxide 150 will serve as tunneloxide on substrate 120, and will provide sidewall insulation for theselect gates. The oxide thickness depends on the dopants and dopantconcentrations. In some embodiments, oxide 150 is 60 Å to 100 Å thick onsubstrate 120, and is 300 Å thick on the select gate sidewalls. Theperipheral area is covered by oxide 810 (FIG. 13B), and remainssubstantially unchanged during this step. Oxide 150 can be nitrided toprevent boron diffusion from floating gates 160 into substrate 120 ifthe floating gates will be doped with boron. In the embodiment beingdescribed, the floating gates will be doped P+ to improve the dataretention time. (The data retention is improved because the P+ dopedpolysilicon is a high work function material. See U.S. Pat. No.6,518,618 issued Feb. 11, 2003 to Fazio et al. and incorporated hereinby reference.)

If desired, an additional Vt adjust implant can be performed into thearray to adjust the threshold voltage of the floating gate transistors(FG/CG transistors). This implant can be performed either before orafter the formation of oxide 150. In one embodiment, the implant isperformed after the etch of polysilicon 140 to define the select gates(FIG. 14A) before the removal of oxide 130 from the FG/CG channel areas.The floating gate transistors can be either enhancement or depletionmode transistors.

Floating gate polysilicon 160 (FIG. 17, cross section X1–X1′) isdeposited over the structure, by LPCVD for example, and is doped P+during or after the deposition. Polysilicon 160 is sufficiently thick toensure that its top surface is at least as high throughout the wafer asthe top surface of oxide 810. In the embodiment of FIG. 17, the topsurface of layer 160 is planar due to a conformal deposition to athickness larger than half the distance between the adjacent select gatelines 140. In one embodiment, the distance between select gate lines 140is 0.8 μm, and the polysilicon 160 is more than 0.4 μm thick.

If the top surface of polysilicon 160 is not planar, it is planarized byCMP or a suitable etch.

After planarization (if needed), layer 160 is etched down without amask. The etch end point is when STI oxide 220 becomes exposed. FIG. 18(cross section X1–X1′) shows an intermediate stage in this etch, whenoxide 810 becomes exposed. At this stage, layer 160 has been removedfrom the periphery, so the periphery becomes as in FIG. 13B. The etchendpoint can be the exposure of oxide 220. The endpoint is well definedif the layer 810 is SiON or silicon nitride, but it is also possible todetect the exposure of oxide 220 if layer 810 is silicon dioxide.Alternatively, the etch can be programmed as a timed etch continuing fora predetermined time after the exposure of layer 810.

FIGS. 19A (cross section X1–X1′) and 19B (cross section Y2–Y2′) show thearray area at the end of the polysilicon etch. The polysilicon has beenremoved from the top surface of oxide 220. In some embodiments, thefinal thickness of layer 160 is 1200 Å. The etch is selective to oxide810.

Optionally, a timed etch of oxide 220 is performed to recess the topsurface of oxide 220 below the surface of polysilicon 160. See FIG. 20A(cross section Y2–Y2′) and FIG. 20B (perspective view of the array).This etch will improve the capacitive coupling between the floating andcontrol gates. See the aforementioned U.S. Pat. No. 6,355,524. In theembodiment of FIGS. 20A, 20B, the oxide 220 continues to protrude abovethe top surface of substrate 120 by about 0.10 μm. In other embodiments,the oxide 220 does not protrude above the substrate after the etch (thetop surface of layer 220 is level with the top surface of the substrateafter the oxide etch).

As mentioned above, layer 810 is sufficiently thick to withstand thisetch.

ONO layer 164 (FIG. 21A, cross section X1–X1′, and FIG. 21B, periphery)is formed over the structure. Control gate polysilicon layer 170 isdeposited on ONO 164 and is doped during or after the deposition. Thislayer is doped N+ in the embodiment being described, P+ in otherembodiments. This may also be a metal or metal suicide layer, or someother conductive material.

The top surface of polysilicon 170 is not planar in the array area.Layer 170 has protrusions 170.1 over the select gate lines 140. Cavities170C form in layer 170 between protrusions 170.1 over the futurepositions of bitline regions 174 (FIG. 30A). The protrusions 170.1 willbe used to define the overlap between the floating and control gateswithout additional dependence on photolithographic alignment.

As shown in FIG. 22 (cross section X1–X1′), a layer 171 0 is depositedover the structure and etched without a mask to expose the polysilicon170. Layer 1710 fills the cavities 170C. When layer 1710 is etched inthe array area, layer 1710 is removed in the periphery, so the peripherybecomes as in FIG. 21B. In one embodiment, layer 1710 is silicon nitridedeposited to have a planar top surface or planarized during the etch.

Polysilicon 170 is etched without a mask. See FIG. 23A (cross sectionX1–X1′) and 23B (periphery). This etch attacks the polysilicon portions170.1 and exposes ONO 164. Polysilicon layer 170 becomes broken over theselect gate lines 140. In other words, the polysilicon etch creates agap 170G (a through hole) in polysilicon layer 170 over each select gateline 140. In the embodiment of FIG. 23A, the etch endpoint is theexposure of ONO 164. In other embodiments, the etch continues after theexposure of ONO 164. In either case, at the conclusion of thepolysilicon etch, polysilicon 170 is exposed near the select gates 140but some of polysilicon 170 is covered by nitride 1710. The width W1 ofthe exposed portions of polysilicon layer 170 adjacent to gaps 170G willdefine the width of the control and floating gates in a self-alignedmanner as illustrated below.

In some embodiments, the minimum thickness of polysilicon 170 (near thegaps 170G) is 0.18 μm, and the width W1 is also 0.18 μm.

In the embodiment of FIG. 23A, the etch of polysilicon 170 is selectiveto nitride 1710. In other embodiments, the etch is not selective to thenitride, and nitride 1710 is etched at the same rate as the polysilicon.The etch can stop on the top oxide sub-layer of ONO 164. The etch can bereplaced with CMP. In some embodiments, the etch or the CMP removes someor all of ONO 164 above the select gates 140 and exposes the oxide 810.In either case, at the conclusion of the etch or the CMP process,polysilicon 170 is exposed near the select gates 140 but some ofpolysilicon 170 is covered by nitride 1710. The width W1 of the exposedpolysilicon portions will define the width of the control and floatinggates as illustrated below.

A protective layer 1910 (FIG. 24, cross section X1–X1′) is formedadjacent to gaps 170G to protect the polysilicon 170 near the selectgates 140. In one embodiment, layer 1910 is silicon dioxide formed bythermal oxidation of layer 170. An exemplary thickness of oxide 1910 is500 Å. Layer 1910 can also be a conductive metal silicide formedselectively on polysilicon 170 by a salicide (self-aligned silicidation)technique. In another embodiment, layer 1910 is deposited over the wholewafer and then removed by CMP from the top surface of layer 1710. SeeU.S. patent application Ser. No. 10/393,212 filed Mar. 19, 2003 by YiDing and incorporated herein by reference.

Nitride 1710 is removed (by a wet etch for example) selectively to oxide1910. The resulting structure is shown in FIG. 25 (cross sectionX1–X1′). The periphery remains as in FIG. 23B.

Polysilicon 170, ONO 164, and polysilicon 160 are etched with oxide 1910as a mask. The resulting structure is shown in FIG. 26A (cross sectionX1–X1′) and FIG. 26B (periphery). In some embodiments, the polysiliconetch of layers 170, 160 is anisotropic, and the etch of ONO 164 isisotropic or anisotropic. The ONO etch may remove the ONO 164 over theselect gates 140 and may also remove portions of oxide 1910 and/or oxide810.

In each FG/CG stack, the floating gate 160 together with control gate170 control the underlying portion of the cell's channel region.

A photoresist layer (not shown) is formed over the wafer and patternedto cover the array but expose the entire periphery. Then oxide 810 (FIG.26B) is etched away from the peripheral area.

The resist covering the array is removed, and another photoresist layer(not shown) is formed to cover the array and define the peripheraltransistor gates. Polysilicon 140 is etched away where exposed by thisresist.

The resist is removed. The wafer is coated with a photoresist layer 2720(FIG. 27B, periphery). The resist is patterned to expose the entirearray area (FIG. 27A, cross section X1–X1′) and also to expose theperipheral NMOS transistor regions. FIG. 27B shows a peripheral NMOStransistor region 512N with a P well 2724P, and a peripheral PMOStransistor region 512P with an N well 2724N. These wells were definedbefore formation of oxide 130. There can be many regions 512N, 512P inthe integrated circuit. Resist 2720 covers the PMOS transistor regions512P. An N type implant (N−) is performed to form the LDD (lightly dopeddrain) extensions for peripheral NMOS source/drain regions 273ON (FIG.27B). This implant also dopes the NMOS gates 140 in the periphery. Inaddition, the implant dopes bitline regions 174 (FIG. 27A).

In some embodiments, the memory array is not exposed by resist 2720, andno doping is performed in the bitline regions at this step.

Resist 2720 is removed, and another photoresist layer 2820 (FIG. 28,periphery) is formed to cover the NMOS peripheral transistor regions512N and the memory array. A P type implant (P−) is performed to formthe LDD extensions for PMOS source/drain regions 2730P and to dope theperipheral PMOS transistor gates.

Resist 2820 is removed. A thin silicon dioxide layer 2904 (see FIG. 29A,cross section X1–X1′, and FIG. 29B, periphery) is grown on the exposedsilicon surfaces of layers 140, 160, 170 by a rapid thermal oxidationprocess (RTO). Alternative techniques can also be used such as chemicalvapor deposition (e.g. TEOS CVD), a high temperature oxide process(HTO), or other suitable techniques, known or to be invented. Thesetechniques may form the oxide 2904 over the entire structure and notonly on the silicon surfaces. An exemplary thickness of oxide 2904 is100 Å.

A silicon nitride layer 2910 is deposited to an exemplary thickness of500 Å to 800 Å. Layer 2910 is etched anisotropically without a mask toform sidewall spacers over the gate structures. The etch of nitride 2910may remove some of oxide 810 in the array area (FIG. 29A). If oxide 2904was deposited over the entire structure (by TEOS CVD or HTO forexample), oxide 2904 will help protect the substrate 120 during thenitride etch.

Then N+ and P+ implants are performed to create source/drain structuresfor the peripheral transistors and the bitline regions 174. Moreparticularly, the peripheral PMOS transistor area 512P is masked withresist (not shown), and an N+ implant is performed to create thesource/drain structures for bitline regions 174 and the peripheral NMOStransistors and increase the dopant concentration in the peripheral NMOSgates 140. The floating, control and select gates and the overlyinglayers mask this implant so no additional masking in the array area isneeded.

The resist is removed. The array and the peripheral NMOS transistorregions 512N are masked with a resist (not shown), and a P+ implant isperformed to create the source/drain structures for the peripheral PMOStransistors and increase the dopant concentration in the PMOS transistorgates 140.

The resist is removed. A silicon dioxide etch is performed to remove theoxide 1910 and expose the control gate lines 170 (FIG. 30A, crosssection X1–X1′). This etch also removes the exposed portions of oxide150 over bitline regions 174 in the array area, the exposed oxide 130over source/drain regions 2730N, 2730P in the periphery (see FIG. 30B),and the oxide 2904 over the peripheral transistor gates.

A conductive metal silicide layer 2920 is formed by a self-alignedsilicidation (salicide) process on the exposed silicon surfaces ofcontrol gate lines 170, bitline regions 174, peripheral transistor gates140 and peripheral source/drain regions 2730N, 2730P. The salicideprocess involves depositing a metal layer, heating the structure toreact the metal with the silicon, and removing the unreacted metal. Thiscan be followed by an anneal or any other suitable processing, known orto be invented, to improve the silicide properties (e.g. increase itsconductivity). Titanium, cobalt, nickel, and other conductive materials,known or to be invented, can be used for the metal layer. Non-salicideselective deposition techniques, known or to be invented, thatselectively form a conductive layer 2920 on the exposed silicon but noton a non-silicon surface, can also be used. Silicide 2920 has a lowerresistivity and a lower sheet resistance than polysilicon 170.

As noted above in connection with FIG. 24, layer 1910 can be aconductive metal silicide formed by a salicide process. In this case,layer 1910 does not have to be removed. The silicidation process of FIG.30A will silicide the bitline regions 174, the peripheral gates 140 andthe peripheral source/drain regions 2730.

As shown in FIG. 31A (cross section X1–X1′), FIG. 31B (array boundary oran array gap without floating gates), and FIGS. 31C and 31D (periphery),inter-level dielectric 3204 is deposited over the wafer. FIG. 31C showsonly an NMOS transistor region, but the PMOS regions are similar.Contact openings are etched in dielectric 3204 to expose the silicidedsurfaces of bitline regions 174 (FIG. 31A), control gates 170 (FIG.31B), peripheral source/drain regions 2730N and 2730P (FIGS. 30B, 31C),and peripheral gates 140 (FIG. 31D). The silicide 2920 protects thebitline regions 174 and the source/drain regions 2730 during this etch.A conductive layer 3210 (e.g. metal) is deposited and patterned tocontact the silicided regions. The figures also show an optional metallayer 3220 (e.g. tungsten) used to fill the contact openings before thedeposition of layer 3210.

In the embodiment of FIG. 31A, metal 3210 is used to form jumpersbetween the adjacent bitline regions 174 connected together (see FIG.9). Then another dielectric layer 3230 (not shown in FIGS. 31B–31D) isdeposited, contact openings are etched in this layer to jumpers 3210,and another metal layer 3240 is deposited on top and patterned to formthe bitlines 180. The bitlines contact the bitline regions 174 throughthe jumpers made from metal 3210. The openings in layer 3240 are filledwith optional tungsten plugs 3250 before the metal 3240 is deposited.

FIG. 31E (top view) shows an extension of a peripheral transistor gate140 over STI oxide 220. The extension can be made to form a contact tothe gate or for some other reason (e.g. to connect the gate to otherfeatures). The region 120 i at the interface between the substrate 120and field oxide 220 is protected from the divot formation because thegate is formed using the first polysilicon layer 140. See also FIG. 13B.The transistor of FIG. 31E can be a high voltage transistor (in area512H in FIG. 12B) or a low voltage transistor (in area 512L).

In FIG. 30A, the width of select gate 140 is shown as Ls, and the widthof each of floating gates 160 is shown as Lf. The floating gate width Lfis defined by the parameter W1 (FIG. 23A) in a self-aligned manner, soLf can be smaller than the minimal photolithographic line width. Ls canalso be smaller than the minimal photolithographic line width asexplained above in connection with FIG. 14A. Ls can be smaller than Lf,or can be equal to or larger than Lf.

In each bit of the memory cell, ONO layer 164 forms a continuous featureoverlying the respective floating gate and overlaying a sidewall ofselect gate line 140. This feature extends the whole length of theselect gate line 140 (in the Y direction). Control gate 170 overlies thecontinuous feature of ONO 164. The portion of ONO 164 overlaying thesidewall of select gate line 140 separates the control gate 170 from theselect gate 140.

Other details of the memory fabrication process for one embodiment aregiven in U.S. patent application Ser. No. 10/393,212 “NONVOLATILEMEMORIES AND METHODS OF FABRICATION” filed Mar. 19, 2003 by Yi Ding andincorporated herein by reference.

FIG. 32 shows an alternative layout of the array. Here the connectionbetween the source/drain regions 174 in the adjacent columns is donethrough the substrate 120. Each contiguous N+ type region 174 providestwo source/drain regions for one of the two adjacent columns and alsoprovides two source/drain regions 174 for the other one of the 10adjacent columns. In the first and last rows of the array, each region174 provides one source/drain region for each of the two adjacentcolumns. Jumpers made from layer 3210 of FIG. 31A are unnecessary. Layer3210 can be used to form the bitlines 180. The number of bitline contactopenings 174C can be reduced, because only one contact is needed foreach pair of source/drain regions 174 that are shorted together. Otherlayouts are also possible.

In some embodiments, the memory cells are read, programmed and erasedusing the same voltages and mechanisms as the cell of FIG. 5. Theprogramming is done by channel hot electro ejection (CHIE) orFowler-Nordheim tunneling. The voltages can be as in FIGS. 6–8. Otherexemplary voltages are shown in the following Table 1:

TABLE 1 Program Read (CHEI) Erase Select gate 140 Selected row: 2.5 V1.5 V 2 V Unselected row:   0 V   0 V 0 V Control gate 170 Selected row:Selected bit 1.5 V to 2 V    9 V to 10 V  −9 V to −10 V (Left or Right):Unselected bit:   7 V to 7.5 V   7 V to 7.5 V 0 V Unselected row:   0 V  0 V 0 V Bitline 180 Selected column: Selected bit: 1.5 V 4.5 V to 5V   Floating Unselected bit:   0 V   0 V 0 V Unselected column:   0 V  0 V 0 V Substrate 120:   0 V   0 V 7 V to 8 V

The erase operation is through the channel region in substrate 120 (bulkerase). In other embodiments, the memory is erased through asource/drain region 174. The programming can be performed byFowler-Nordheim tunneling. In some embodiments, the programming isperformed by an electron transfer between floating gate 160 and selectgate 140.

The invention is not limited to any particular read, erase orprogramming techniques, or to particular voltages. For example, thememory can be powered by multiple power supply voltages. Floating gates160 can be defined using a masked etch, and can extend over sidewalls ofselect gate lines 140. See U.S. patent application Ser. No. 10/411,813filed by Yi Ding on Apr. 10, 2003 and incorporated herein by reference.The invention is not limited to particular conductivity types. Selectgates 140 may include non-semiconductor materials (e.g. metal silicide).The invention is not limited to the arrays of FIG. 9. Also, substrateisolation regions 220 do not have to traverse the entire array. Theinvention is applicable to non-flash memories (e.g. non-flash EEPROMs)and to multi-level memory cells (such a cell can store multiple bits ofinformation in each floating gate). Other embodiments and variations arewithin the scope of the invention, as defined by the appended claims.

1. A method for manufacturing an integrated circuit comprising a nonvolatile memory cell having source/drain regions of a first conductivity type in a semiconductor substrate and having a channel region in the semiconductor substrate between the source/drain regions, the method comprising: forming a first conductive gate comprising a semiconductor material of a second conductivity type opposite to the first conductivity type, the first conductive gate overlying a portion of the channel region; and forming a floating gate overlying a portion of the channel regions; wherein the channel region comprises a surface region underlying the first conductive gate and having a lower net dopant concentration of the second conductivity type than a region immediately below the surface region.
 2. The method of claim 1 wherein the first conductive gate is a gate of a buried channel transistor.
 3. The method of claim 1, wherein the surface region is at most 0.2 μm deep.
 4. The method of claim 1 further comprising implanting an impurity of the first conductivity type into surface region.
 5. The method of claim 4 wherein the surface region is at most 0.2 μm deep.
 6. The method of claim 1 wherein the channel region has the second conductivity type.
 7. The method of claim 1 wherein the first conductive gate is to turn on the underlying portion of the channel region to provide access to the nonvolatile memory cell.
 8. The method of claim 1 wherein the floating gate is one of two floating gates of the nonvolatile memory cell, each floating gate overlying a portion of the channel region.
 9. The method of claim 1 wherein the first conductivity type is type N.
 10. The method of claim 1 wherein the first conductivity type is type P.
 11. A method for manufacturing an integrated circuit comprising a nonvolatile memory cell having source/drain regions of a first conductivity type in a semiconductor substrate and having a channel region in the substrate between the source/drain regions, the method comprising: forming a first conductive gate overlying a portion of the channel region; and forming a floating gate overlying a portion of the channel region; wherein the first conductive gate is a gate of a buried channel transistor; wherein the channel region comprises a surface region underlying the first conductive gate and having a lower net dopant concentration of the second conductivity type than a region immediately below the surface region.
 12. The method of claim 11 wherein the surface region is at most 0.2 μm deep.
 13. The method of claim 11 further comprising implanting an impurity of the first conductivity type into the surface region.
 14. The method of claim 13 wherein the surface region is at most 0.2 μm deep.
 15. The method of claim 11 wherein the channel region has the second conductivity type.
 16. The method of claim 11 wherein the first conductive gate is to turn on the underlying portion of the channel portion to provide access to the memory cell.
 17. The method of claim 11 wherein the floating gate is one of two floating gates of the memory cell, each floating gate overlying a portion of the channel region.
 18. The method of claim 11 wherein the first conductivity type is type N.
 19. The method of claim 11 wherein the first conductivity type is type P. 